The invention relates to a method for acquiring measured values in electronic analog circuits having at least one measurement point, in particular safety-relevant circuits for passenger protection systems in motor vehicles, and a circuit arrangement for performing the method according to the invention.
In the case of passenger protection systems in motor vehicles comprising human protection devices such as airbags, belt tighteners and suchlike, a very high degree of system reliability is required. In particular, in the case of safety-critical electronic circuits, it is required that these are always ready to function and that if a fault occurs it will be displayed immediately because the user of a vehicle can immediately arrange for the passenger protection system to be inspected and repaired.
In airbag or belt-tightener systems, therefore, electrical and/or electronic components of the system are tested for proper functioning in the course of a self-diagnostics routine performed by the system electronics. This necessitates the implementation of additional circuit units in the electronic components of the system in order to generate test signals and to acquire measured values.
Thus, after the onboard power supply of the motor vehicle has been switched on, a comprehensive self-test is performed and also cyclic self-tests while the motor vehicle is in operation, where the comprehensive self-test that is performed when switching on the power supply serves as a basis for the subsequent cyclic self-tests.
The self-test of analog switching functions of the electronic components of a passenger protection system is particularly critical here on account of the wide variety of faults. Apart from the switching function as such, parametric errors or range tolerances must also be recorded. If such faults are to be registered, it is necessary to be able to stimulate the electronic components of such passenger protection systems accordingly and also to have the means available of performing analog measurements.
In existing safety systems, relatively elaborate A/D converters are used for this purpose to which the relevant measured quantities are offered via analog multiplexers. This measured quantity is then converted into a digital value and evaluated in a digital arithmetic unit (microprocessor).
As a rule, this digital arithmetic unit is normally also responsible for controlling the stimulation of the electronic components in order to generate appropriate measured quantities and for controlling the multiplexers.
Apart from the expense of applying analog multiplexers, considerable circuitry must usually be implemented for matching the signal to the input range of the A/D converter. Influencing factors such as various operating temperatures or crosstalk from other modules, especially when signals are transmitted beyond the individual components, must also be taken into consideration.